NAIROBI, KENYA (CBS/AP) -- Sporadic gunfire and explosions were heard Monday from the shopping mall in Kenya's capital where Somali terrorists remained holed-up with an unknown number of hostages.

The standoff began around midday on Saturday, when an estimated 10 to 15 militants from the al Qaeda-linked group al-Shabab stormed the mall, shooting indiscriminately and hurling grenades inside the large shopping center popular with Westerners. At least 68 people have been killed and another 175 injured according to Kenyan officials.

With the mall surrounded by Kenyan police, soldiers special forces, a spokesman for al-Shabab released an audio statement online warning that the remaining hostages would be killed if the security forces attempted to end the standoff by force.

Police inspector general David Kimaiyo said Monday on his Twitter feed that security forces were "closing in" on the militants in the mall, according to the Reuters news agency, but he did not go into detail on the operations. He said several more hostages had been rescued.

"Israelis and Kenyan forces have tried to enter Westgate (mall) by force but they could not, the mujahideen (fighters) will kill the hostages if the enemies use force," a man identifying himself as Sheikh Ali Mohamud Rage said in the clip, according to the Reuters news agency. CBS News could not independently verify the authenticity of the audio statement, but it was posted on a website often used by the Somali terror group.

Westgate Mall is at least partially owned by Israelis, and reports circulated that the Kenyan forces were getting advice on the ground from both Israeli and U.S. military officials. There was no indication that U.S. forces were involved in the military operation.

A senior Israeli defense official said there were no Israeli forces participating in an assault, but the official said it was possible that Israeli advisers were providing assistance.

A Kenyan military spokesman said Monday the fate of hostages inside the upscale mall was not clear, despite earlier claims that most hostages had been rescued. Many civilians were seen leaving the mall over the course of Saturday, having escaped unaided, but it was unclear how many remained inside.

CBS News correspondent Charlie D'Agata arrived Monday morning to Nairobi and said the city was tense. Even staff at the hotel in the capital city were taking additional precautions, searching bags and scrutinizing visitors more carefully than normal. Gunfire had been heard sporadically Monday morning, and security forces had established a roughly 200 yard perimeter around the Westgate mall. Helicopters hovered overhead.

Kenyan police said late Sunday on Twitter that a "MAJOR" assault by security forces was ongoing to end the siege. "This will end tonight. Our forces will prevail. Kenyans are standing firm against aggression, and we will win," Kenya's Disaster Operations Centre said on Twitter.

kenya, westgate, mall, hostage, attack, al-shabab Soldiers from the Kenya Defense Forces carry a wounded colleague, following the sound of explosions and gunfire, out of the Westgate Mall in Nairobi, Kenya Sunday, Sept. 22, 2013. / AP PHOTO/BEN CURTISKenya Defence Forces later said it had rescued most hostages and had taken control of most of the mall. Officials did not immediately release the number of hostages rescued or the number that remained. Four Kenyan military personnel were wounded in the operation, the military said.

It is not the first time terror has struck in Kenya. In 2002, militants bombed an Israeli-owned luxury hotel near Mombasa, killing 13 people, and tried to shoot down an Israeli airliner at the same time. Kenyans and foreigners were among those confirmed dead, including British, French, Canadians, Indians, a Ghanaian, a South African and a Chinese woman.

Speaking by phone to the BBC on Monday, another al-Shabab spokesman identifying himself as Abu Omar denied reports that U.S. nationals and women were among the militants involved in the attack on the mall. Al-Shabab has drawn a significant number of young Somali-American men from the U.S. as recruits, but there was no confirmation from U.S. officials as of Monday morning regarding involvement in the Westgate attack by U.S. nationals.

U.S. Secretary of State John Kerry condemned what he called "an enormous offense against everybody's sense of right and wrong," and called the attackers "ruthless and completely reckless terrorists."

Kerry, who was in New York for meetings at the United Nations, spoke Sunday with Somalia's foreign minister and U.N. ambassador.

"We went as a nation into Somalia to help stabilize the country and most importantly to fight terror that had been unleashed on Kenya and the world," said Kenyatta."We shall not relent on the war on terror."

He said although the violent attack had succeeded, Kenyan security forces had "neutralized" many others.

Earlier in the day Kenyatta said he his nephew and his nephew's fiance were killed in the attack.

Kenyan security officials sought to reassure the families of hostages inside but implied that hostages could be killed. The security operation is "delicate" because Kenyan forces hoped to ensure the hostages are evacuated safely, said Interior Cabinet Secretary Joseph Lenku.

"The priority is to save as many lives as possible," Lenku said, adding that more than 1,000 people escaped the attack inside the mall on Saturday.

"We have received a lot of messages from friendly countries, but for now it remains our operation," Lenku said.

CBS News' travel editor Peter Greenberg, who is visiting Nairobi, described the Westgate as "a very large mall, with more than 80 stores and large grocery stores and banks," that is popular with locals and tourists alike.

Britain's prime minister, confirming the deaths of three British nationals, told the country to "prepare ourselves for further bad news." There are approximately 40,000 British nationals living in Nairobi.

Kenyans and foreigners were among those confirmed dead, including British, French, Canadians, Indians, a Ghanaian, a South African and a Chinese woman.

Kenya's Red Cross said the death toll on Sunday rose to 68 after nine bodies were recovered in a joint rescue mission.

Britain's Foreign Office said that Foreign Secretary William Hague has chaired a meeting of Britain's crisis committee and sent a rapid deployment team from London to Nairobi to provide extra consular support.

The United Nations Security Council condemned the attacks and "expressed their solidarity with the people and Government of Kenya" in a statement.

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